The present invention relates generally to methods and apparatus for debridging solder joints and the like, and more particularly to the debridging of microelectronic solder joints using a laser.
In the microelectronics industry, surface mounted microelectronic circuit devices and the like are typically mounted on a substrate such as a printed wire board by soldering the leads of the devices to pads bonded to the board surface.
The formation of an undesired solder bridge between two or more adjacent soldered leads has been a longstanding problem in the microelectronics industry. Since such bridges represent undesired electrical connections, it is necessary that they be removed. There is an emphasis today on minimizing the physical size of microelectronic circuits and on increasing the physical density of circuits on chips. As the size of the microelectronic circuit leads and the pitch (center-to-center spacing) of the leads becomes smaller, it becomes even more difficult to perform a debridging operation. This is because of the large size of conventional tools relative to the size of the leads and to the fact that the leads generally are quite flexible and easily moved and become particularly acute with leads having a width of the order of 4 mils or less and a pitch of the order of 8 mils or less, which is typical of tape automated bonding (TAB) devices and is similar to other surface mounted devices.
In the past, both conductive and convective heating methods have been used for debridging and, in some cases, in desoldering operations. Conductive methods have included the use of soldering irons to melt the excess solder, which is subsequently removed by a wicking action onto another material or by evacuating the excess solder away from the area. This method may be effective with larger 25 mil pitch leads or greater, but the large physical size of a soldering iron and the inability to confine the area affected by it to the undesired solder bridge makes it very difficult to use a soldering iron with 8 mil pitch leads. Convective heating methods include the use of a hot air jet for melting solder, and such methods are presently being used for desoldering operations. Although it may be possible to perform a debridging operation using an individual hot air jet, it is difficult to control such an air jet and to limit the area which is heated. This makes it difficult to control the amount of solder which is removed, especially at a solder joint, and makes it particularly difficult to use such a process with 8 mil pitch or smaller leads. In order to melt and remove just the solder that constitutes the bridge between 8 mil pitch leads (typically a 4 mil gap) without effecting the soldered leads directly is very difficult when using a hot air jet. Moreover, 8 mil pitch leads are quite flexible and fragile, as noted, making it very easy for a jet of air to lift the lead off of its pad or to damage the lead during debridging. A similar problem exists with respect to a soldering iron which contacts the lead and exerts a force on it.
At present, there is, in some cases, no way to repair microelectronic circuits having such small pitch leads and bridges between solder joints. Where repair is possible, it is usually very difficult and expensive. Therefore, in a production environment, if a microelectronic circuit board is found to have a solder bridge, the absence of a convenient and inexpensive way to repair the board usually results in the board being discarded rather than an attempt being made to repair it.
There is a need for a method and an apparatus for repairing microelectronic circuit boards and the like to remove bridges quickly and easily between solder joints or solder leads without affecting the integrity of the debridged joints or causing damage to the leads. It is to these ends that the present invention is directed.